1. Field of the Invention
The present invention relates to a power train of automatic transmission, and more particularly to a power train of automatic transmission adapted to initiate a shift stage of five forward speeds and one reverse speed with two planetary gear sets and seven friction elements for compactness, lightness and decreased power loss at three forward speeds.
2. Description of the Prior Art
Generally, an automatic transmission is mounted with a transmission control unit for automatically controlling a transmission ratio according to running speed of a vehicle and load change. The transmission control unit controls a plurality of friction elements mounted at a power train in operation or inoperation state to adjust revolution at an output terminal by selecting any one of three elements at the planetary gear sets (sun gear, ring gear and planetary carrier) as an input element and the others as reaction force element.
Actually, in order to design a power train for outputting a transmission ratio on four forward speeds and one reverse speed, a combined planetary gear set combining two simple planetary gear sets and at least five friction elements should be used.
Furthermore, the combined planetary gear set has planetary carriers directly connected for at least two different elements to be commonly used, or for a sun gear to be commonly used, or for planetary carrier and ring gear to be directly connected, or for the planetary carrier and the ring gear to be directly and commonly connected in direct structure, possessing four dependent actuating elements.
However, there is a problem in the power train thus described in that its engine performance cannot be fully utilized in a recent trend of high powered engine although a shift stage of four forward speeds and one reverse speed can be obtained. In other words, the automatic transmission cannot achieve good fuel consumption rate, power and driving performances. Lately, as a means for fully utilizing an engine performance at a high-powered engine, a power train having a shift stage of five forward speeds and one speed has been disclosed for multi-staging the transmission ratio. A power train according to the prior art proposed for a shift stage of five forward speeds and one reverse speed needs three simple planetary gear sets and six friction elements, or a power train that requires three simple planetary gear sets and seven friction elements has been provided.